Prince William and Prince Harry’s relationship has gone completely off the rails, as the royal brothers are in a rift that is showing no signs of ending anytime soon. Robert Lacey, the royal expert who wrote the book “Battle of Brothers: William and Harry: The Inside Story of a Family in Tumult,” is speaking out this week to discuss the feud that has separated these two brothers like never before.

“The rift gets more serious with every passing day,” Lacey told Page Six, adding that William and Harry’s relationship with one another is at an all-time low.

“The chickens have come home to roost,” said Lacey, who acts as a historical consultant for “The Crown” on Netflix. “We thought the disorder, chaos and tragedy of the ’80s and ’90s involving their parents were over. Now [it’s all] come back to life in the shape of these two boys. They are the legacy of all that heartache.”

He was referring to the marriage and subsequent divorce of the brothers’ parents, Princess Diana and Prince Charles, and to their mother’s death in 1997.

“They both talk about the importance of mental health and acknowledge they are psychologically fragile,” Lacey said. “But they coped with it in different ways.”

William was given advice by his grandmother, Queen Elizabeth, and found strength in the fact that he would be king someday. Lacey explained that as a result of this, “When he meets Kate Middleton at university, he actually makes the poor young woman wait nine years to effectively audition her for the job as wife to the head of state. He took the lesson of duty.”

Lacey added that Harry learned “opposite” lesson: that he was the victim of “the arranged marriage” between virginal Diana and the worldlier Charles that “swiftly became loveless.”

“His attitude was: ‘Well, I’m not going to make the same mistake as my parents,’” the royal author said. “ ‘I’m marrying for love.’ And he’s been so happy and proud to proclaim he loved Meghan virtually from the moment he met her in 2016. That impulse has guided him.”

Lacey sees it as a “tragedy” that Meghan Markle is no longer part of the royal family. For this, he blames the stuffy occupants of Buckingham Palace who didn’t like it that Harry and Meghan were swiftly becoming “rock-star royals” who overshadowed William and Kate.

“Everyone thought Harry was going to marry a nice girl called Henrietta or Annabel and go live in the countryside,” Lacey said. “Instead, he finds Meghan, full of American piss and vinegar, energy and independence of thought. If you bring into the old-fashioned, creaking structure of the House of Windsor someone who is a crusader, dedicated to women’s rights and social change, she isn’t going to give up those qualities. When she came over to Britain, she thought it was a new platform. Whereas the royal family didn’t see themselves as offering her a platform at all.”

Lacey said that Harry was appalled that his family was unwilling to open their minds to Meghan. The straw that broke the camel’s back was when the Queen gave her 2019 Christmas broadcast with no portrait of Meghan, Harry or their son Archie in view.

“Who does or does not feature on the royal Christmas desk has always been like the changing panorama of faces on the historic balcony of Moscow’s Kremlin,” Lacey added. “It showed who was in favor and who was not — and, at Windsor in December 2019, this even extended to babies.”

He went on to say that these days, the rift between William and Harry is wider than ever.

“It’s looking less and less likely for a reconciliation,” Lacey said. “I don’t see a way back in for Harry, not into the working royal family.”